1. Field of the Invention
This invention relates to the use of a Class C fly ash in conjunction with silica fume in cement compositions, more especially cement compositions which are to be employed with alkali-reactive aggregates, and cement compositions which are to be employed in environments in which sulphate resistance is required.
2. Description of Prior Art
Mineral aggregate is employed in concrete, the binder for the concrete being Portland cement. It is known to replace a portion of the Portland cement with a pozzolan such as fly ash, which is the finely divided residue produced in the combustion of ground or powdered coal.
There are two principal classes of fly ash, namely Class F and Class C, and these are defined by ASTM C 618, which is incorporated herein by reference. Class F and Class C fly ash differ in the nature of the coal from which they are derived. Class F fly ash has a very low analytical content of lime (CaO), typically below 8%, by weight, whereas Class C fly ash typically has a high analytical content of lime, above 10%, by weight.
A particular problem arises with the use of certain classes of mineral aggregate in concrete. Some aggregates exhibit an alkali aggregate reaction (AAR), and especially an alkali-silica reaction (ASR), in which reactive silica in the aggregate reacts with alkalis in the Portland cement during hydration of the cement. The alkalis raise the pH of the interstitial water between particles of the cement, and this results in hydrolysis of the reactive silica with formation of an alkali silicate gel, based on sodium and potassium silicate. These silicates are very hygroscopic and increase in volume as water is absorbed from the interior of the concrete and ultimately from the exterior environment. If the internal pore volume of the concrete is insufficient to accommodate the extra volume of gel, excessive internal pressures, more especially tensile stress, are generated, leading to crack formation in the concrete. The formation of cracks permits ingress of water, carbon dioxide and chlorides, which further accelerate deterioration of the concrete.
Previously it has been found that Class F fly ash, ground granulated blast furnace slag, silica fume and calcined clays such as metakaolin are able to reduce alkali silica reaction in concrete. The siliceous component of these materials reacts with the calcium hydroxide produced in the hydration of Portland cement, to form additional calcium silicate hydrate which is the primary strength forming product of cement hydration. This additional calcium silicate hydrate has a low CaO/SiO.sub.2 ratio and a higher capacity to bind alkalis thereby preventing the attack on aggregates; the pH of the interstitial water also decreases.
Concretes exposed to ground water containing sulphates exhibit deterioration as a result of reaction between the sulphate and hydrated compounds of the hardened cement, especially calcium aluminate hydrate, with formation of calcium sulphoaluminate (ettringite). Crystallization growth occurs with the formation of the calcium sulphoaluminate, resulting in expansion forces within the concrete which can result in cracking and disintegration.